1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a fixing device of an image forming apparatus such as a copying apparatus or a printer employing electrophotographic or electrostatic recording method, a rotary member employed in such image heating apparatus and a method for producing the rotary member.
2. Related Background Art
As the heat fixing apparatus, there has conventionally been employed apparatus of heat roller type or film heating type. In particular, the Japanese Patent Application Laid-open Nos. 63-313182, 2-157878, 4-44075 and 4-204980 propose a method, more specifically a heat fixing method of film heating type in which a toner image on a recording material is fixed across a film between a heater portion and a pressure roller, of not supplying the heat fixing apparatus with an electric power at the stand-by state thereby minimizing the electric power consumption.
The configuration of the heat fixing apparatus of film heating type is known in a method of conveying a film in cooperation with a pressure roller under a tension applied by a conveying roller exclusive for film conveying and an idler roller, and a method of driving a cylindrical film by the conveying force from a pressure roller, wherein the former provides an advantage of maintaining secure conveying ability for the film while the latter provides an advantage of realizing a fixing apparatus of low cost resulting from a simpler configuration.
As a specific example, FIG. 2 shows, in a schematic lateral cross-sectional view, a fixing apparatus of the film heating type of the aforementioned latter configuration in the pressure roller driving method, and FIG. 3 is a magnified cross-sectional view of the principal part.
In such apparatus, there are provided a heating member (hereinafter represented as heater) 11 fixedly supported by a heater holder (support member) 12, and an elastic pressure roller 20 maintained in pressure contact with the heater 11 thereby forming a nip portion (fixing nip) N of a predetermined nip width across a heat-resistant thin film (hereinafter represented as fixing film) 13.
The heater 11 is heated and maintained at a predetermined pressure by electric current supply.
The fixing film 13 is a rotary member (rotation body) for heat fixing, which is composed of a cylindrical thin member conveyed in a direction, indicated by an arrow, in sliding contact with the surface of the heater 11 at the fixing nip N by the rotary driving force of the pressure roller 20.
In a state where the heater 11 is heated and maintained at the predetermined temperature and the fixing film 13 is moved in the direction indicated by the arrow, when a recording material P bearing an unfixed toner image t is introduced, as a material to be heated, into the fixing nip N between the fixing film 13 and the pressure roller 20, the recording material P is maintained in close contact with the surface of the fixing film 13 and is conveyed in a sandwiched state in the fixing nip N together with the fixing film 13.
In the fixing nip N, the unfixed toner image on the recording material P is heated by the heater 11 across the fixing film 13 and is thermally fixed as a permanent image on the recording material P.
Having passed the fixing nip N, the recording material P is peeled from the surface of the fixing film 13 and is further conveyed.
The heater 11, constituting the heating member, generally includes a ceramic heater, which will be further clarified with reference to FIGS. 3 and 4.
It is formed, on a surface (opposed to the fixing film 13) of an electrically insulating ceramic substrate 11a of a high thermal conductivity and a low heat capacity, by forming a heat-generating resistor layer 11b such as of silver palladium (Ag/Pd) or Ta2N for example by screen printing along the longitudinal direction of the substrate (perpendicular to the plane of FIG. 3) and covering the surface bearing the heat-generating resistor layer with a thin glass protective layer 11c. In such ceramic heater 11, by passing an electric current through the heat-generating resistor layer 11b, such heater generates heat to rapidly elevate the temperature of the entire heater including the ceramic substrate 11a and the glass protective layer 11c. The temperature increase of the heater 11 is detected by temperature detecting means 14 provided on the rear surface of the heater and is fed back to an current control unit (not shown), which controls the current supply to the heat-generating resistor layer 11b in such a manner that the heater temperature detected by the temperature detecting means 14 is maintained at a predetermined substantially constant temperature (fixing temperature). In this manner the heater 11 is heated and maintained at the predetermined fixing temperature.
The fixing film 13, constituting the rotary member for heat fixing, is made with the thickness of 20 to 70 xcexcm, in order to efficiently transmit the heat from the heater 11 to the recording material P to be heated, in the fixing nip N. The fixing film 13 has a three-layered structure including of a base film layer, a primer layer and a releasing layer. The base film layer is positioned at the side of the heater 11, while the releasing layer is positioned at the side of the pressure roller 20.
The base film layer is composed of polyimide, polyamidimide, PEEK etc. having a higher insulation than the glass protective layer 11c of the heater 11, and has heat resistance and a high elasticity. Also the base film layer maintains the mechanical strength such as tear strength of the entire fixing film. The primer layer is formed with the thickness of 2 to 6 xcexcm. The releasing layer is provided for preventing toner offsetting to the fixing film, and is formed by coating fluorinated resin such as PFA, PTFE or FEP with a thickness of about 10 xcexcm.
The heater holder 12 is for example formed by a member of heat resistant plastics and serves as a conveying guide for the fixing film, as well as supporting the heater.
In such heating apparatus of the film heating type utilizing a thin fixing film, because of the high rigidity of the ceramic heater 11 constituting the heating member, the pressure roller 20 having an elastic layer 22 is pressed to the flat lower surface of the heater 11 to form the fixing nip portion N of a predetermined width, and quick-starting heat fixation is realized by heating the fixing nip portion N only.
In the above-described configuration, the positional relationship between the heat-generating resitor layer 11b of the heater 11 and the pressure roller 20 will be explained with reference to FIG. 4, in which the longitudinal width W of the heat-generating resistor layer 11b of the heater 11 is formed somewhat narrower than the width D of the elastic layer 22 of the pressure roller 20 maintained in contact across the fixing film 13. Such relationship is adopted in order to prevent breakage of the heater 11 by a thermal stress resulting from local temperature rise therein, in case the heat-generating resistor layer 11b is wider than the pressure roller 20.
Also the heat-generating resistor layer 11b is formed with such a width sufficiently wider than the conveying area of the recording material P bearing the toner image t. It is thus made possible to avoid the influence of temperature decrease in the end portions (caused by heat leakage to the electrical contacts, connectors etc. at the heater ends), thereby realizing satisfactory fixing performance over the entire surface of the recording material. It is also possible to form the heat-generating resistor layer narrower in the end portions of the sheet passing area to increase the heat generation in such end portions, thereby enhancing the fixing performance therein.
Thus, the heat generated by the electric current supply in the heat-generating resistor layer 11b of the heater 11 is efficiently supplied to the recording material P conveyed between the fixing film 13 and the pressure roller 20, and serves to fuse and fix the toner image t on the recording material P.
A symbol S indicates a standard or reference position for the conveying operation of the recording material. The present example is a center-reference apparatus in which the reference position is provided at the center, in the longitudinal direction, of the conveying area of the recording material in the main body of the image forming apparatus.
Also as shown in FIG. 4, on the rear surface of the heater, there are maintained in contact a temperature detecting element 14 such as a thermistor and a thermo protector 15 such as a temperature fuse or a thermo switch, for shutting down the current supply to the heat-generating resistor layer 11b of the heater 11 in case the operation becomes uncontrollable. These elements are positioned within the conveying area of the recording material of a smallest width, that can be conveyed in the image forming apparatus.
The temperature detecting element 14 is provided in the conveying area of the recording material of the smallest width, in order to achieve heat fixation of the toner image on the recording material without defective fixation or high temperature offsetting, even in case the recording material of the smallest width conveyable in the main body of the image forming apparatus is conveyed.
Also the thermo protector 15 is provided in the conveying area of the recording material of the smallest width, in order to prevent erroneous shut-down of the current supply in the normal conveying operation, by overheating in the non-conveying area having a smaller heat resistance than in the conveying area in case the recording material of the smallest width is conveyed.
On the other hand, in the case that the thermo protector 15 is in contact with the rear surface of the heater, there may result a situation where the heat generated in the heat-generating resistor layer 11b is taken away by the thermo protector 15 and cannot be sufficiently given to the recording material, thereby inducing insufficient fixation at the contact position of the thermo protector. In order to avoid such phenomenon, the heat-generating resistor layer 11b of the heater 11 is made partly somewhat narrower as shown in FIG. 4 to increase the resistance of the contact position a in comparison with other portions, thereby ensuring sufficient heat generation. In this manner the amount of heat supply to the recording material is made uniform over the entire longitudinal direction, thereby realizing satisfactory heat fixation without unevenness in the fixing performance.
As the temperature detecting element 14 is also maintained in contact with the rear surface of the heater, it is conceived that the heat generated by the heat-generating resistor layer 11b is similarly taken away by the temperature detecting element 14, but the amount of heat taken away from the heater can be made small by selecting a temperature detecting element of a low heat capacity such as a chip thermistor. Therefore, uniform fixation can be realized without deteriorating the fixing uniformity for the recording material in the longitudinal direction, without adopting a countermeasure similar to the above explanation explained for the thermo protector 15.
The heat fixing apparatus of the film heating type explained in the foregoing has various advantages such as electric power saving and elimination of waiting time for the user because the preliminary heating during stand-by state can be dispersed with owing to the high heating efficiency and the possibility of quick start. In particular, the configuration of driving the cylindrical film 13 with the conveying force of the pressure roller 20, being capable of achieving a low cost, has started to be introduced into the compact low-speed apparatus and is expected to be hereafter adopted in the large high-speed apparatus.
For achieving such higher speed fixation, sufficient thermal energy for fixation has to be supplied even to the recording material P having a shorter passing time through the fixing nip portion N. For achieving such objective, there can be conceived a method of further elevating the fixing temperature, a method of increasing the pressure between the pressure roller 20 and the fixing film 13 thereby increasing the width of the fixing nip serving as the heating area, or a method of changing the material of the heater substrate 11a or the fixing film 13 to another with a higher thermal conductivity thereby increasing the supply amount of heat.
However, such changes increase the burden on the fixing film 13, constituting the heat fixing rotary member, thereby leading to a drawback of accelerating the deterioration thereof and reducing the service life thereof.
For example, if the amount of filler of high thermal conductivity such as BN (boron nitride) or ALN (aluminum nitride) is increased in order to improve the thermal conductivity of the base film layer of the fixing film, the flexibility or strength of the resinous material itself such as polyimide (PI) resin is deteriorated whereby the abrasion or deterioration of the fixing film 13 is accelerated.
For this reason, it is newly conceived to adopt a metal, which is superior in thermal conductivity to resinous materials, as the base layer of the fixing film 13 constituting the heat fixing rotary member.
When used as the heat fixing rotary member, a metal sleeve, because of the thermal conductivity of the constituent material, is capable of transmitting sufficient thermal energy for fixation to the recording material even without relying on a higher fixing temperature or increasing the pressure for widening the fixing nip, thereby realizing a fixing apparatus of film heating type adaptable to a higher speed.
Such metal base layer, however, is revealed to result in the following problem, if the metal base layer has a straight shape in which the external diameter at the center is equal to that at both ends.
In case the metal base layer has a straight shape, because the external diameter thereof at the center is equal to that at both ends, the conveying speed of the recording material P in passing the fixing nip portion N becomes the same as one at the central position of the recording material and at the end positions thereof in the conveying direction, thereby generating wrinkles (crease) or uneven gloss in the recording material.
In consideration of the foregoing, an object of the present invention is to provide an image heating apparatus of a low electric power consumption, adaptable to high speed heating, and a rotary member adapted for use in such apparatus.
Another object of the present invention is to provide an image heating apparatus capable of suppressing wrinkle generation in the recording material, and a rotary member adapted for use in such apparatus.
A further object of the present invention is to provide a method for producing a flexible metal rotary member of a high production yield.
A further object of the present invention is to provide an image heating apparatus comprising a heater, a rotary member having flexibility and adapted to rotate in contact with the heater, the rotary member including a metal base layer, and a back-up member for forming a nip portion in cooperation with the heater across the rotary member, wherein the rotary member includes an area in which the external diameter increases toward the ends in the longitudinal direction of the rotary member.
A further object of the present invention is to provide a rotary member adapted for use in an image heating apparatus, the rotary member comprising, a metal base layer having flexibility, wherein the rotary member includes an area in which the external diameter increases toward the ends in the longitudinal direction.
A further object of the present invention is to provide a method for producing a rotary member, comprising, a step of ironing a metal pipe having flexibility, a step of subjecting the metal pipe subjected to the ironing step to plastic working thereby forming an area in which the external diameter increases toward the ends in the longitudinal direction, and a step of sintering a resin layer on the surface of the metal pipe subjected to the plastic working.
Still another object of the present invention is to provide a method for producing a rotary member, comprising a step of ironing a metal pipe having flexibility thereby forming an area in which the external diameter increases toward the ends in the longitudinal direction, and a step of sintering a resin layer on the surface of the metal pipe subjected to the ironing.
As still further object of the present invention is to provide a method for producing a rotary member, comprising, a step of ironing a metal pipe having flexibility, a step of subjecting the metal pipe subjected to the ironing step to annealing, a step of sintering a resin layer on the surface of the metal pipe subjected to the annealing, and a step of subjecting the metal pipe subjected to the formation of resin layer to plastic working thereby forming an area in which the external diameter increases toward the ends in the longitudinal direction.
These and other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description which is to be taken in conjunction with the accompanying drawings.